Various solar energy collecting apparatus are known which use a reflector for concentrating solar energy at a point or in a region where it can be absorbed by a collector for conversion into useful energy.
The primary problem facing such devices is to efficiently collect energy during daylight hours as the sun moves across the sky. Adding to the problems associated with daily movment of the sun is the fact that the course of the sun's movement across the sky is effected by seasonal changes.
Various methods of tracking the movement of the sun across the sky have been developed for solar energy collectors. In some devices, particularly those with a parabolic cone shaped concentrator, such as that shown in U.S. Pat. No. 670,916 to Eneas, the concentrator and energy collecting device both move to follow the path of the sun's movement. Such devices require elaborate and expensive tracking systems which make them impractical for collecting energy. In another type of solar energy concentrating device, a stationary concentrator and collector are provided which may form a roof structure. As shown in U.S. Pat. No. 4,291,679 to Kersavage, a hyperbolic paraboloid concentrator forms the southern portion of a roof and focuses light on a curved absorber. To compensate for seasonal changes in the course of the sun's movement, the Kersavage patent shows the use to two differently oriented reflectors. The expensive, custom designed roof structure disclosed in Kersavage does not lend itself to widespread use of the solar energy concentrating device disclosed therein.
If a straight absorber is used with a hyperbolic paraboloid concentrator, as shown in U.S. Pat. No. 4,035,064 to Cowman, the desired concentration of light occurs only along portions of the absorber with much of the reflected light being diffused prior to reaching the absorber. While the Cowman device eliminates problems associated with moving either the concentrator or the collector, the quantity of energy absorbed per unit of concentrator surface, or efficiency of the collector, is significantly less. Also, the quantity of light reflected by the concentrator during the early morning and late afternoon is very limited because only a small corner of the reflector is directed toward the sun at those times.
In U.S. Pat. No. 4,111,360 to Barr a solar energy concentrating and collecting arrangement is provided by forming a concave cylindrical concentrator as part of the roof of a building and pivotably supporting a collector above the roof. The collector is movable in response to changes in the location of the focus zone of the concentrator caused by changes in the angle of incidence of sunlight on the concentrator. One problem with this cylindrical concentrator is that the daily time period for efficient energy collection is severly limited since a significant portion of morning and afternoon light is reflected from the concentrator to a location east or west of the collector.
Another problem with the structure disclosed in Barr is that the surface does not have the membrane strength realized by a hyperbolic or otherwise non-developable surface. Thus a specially constructed and reinforced roof structure is required to support the concentrator surface and maintain its shape.
Other cylindrical concentrators with movable collector systems such as that disclosed in U.S. Pat. No. 3,868,823 to Russell, Jr. minimize this loss of light by extending the length of the solar collector. However, due to size limitations such a solution is not feasible if the solar concentrator is to be incorporated into a building.
Therefore, prior art devices fail to provide a highly efficient yet economical solar collector wherein a concentrating surface is formed as part of a building to reflect light into a narrow zone of concentration. Due to the daily and seasonal changes in the angle of incidence of sunlight it is important that the collector moves with the zone of concentration. It is preferred that the zone of concentration is a linear area so that a straight collector may be used to absorb the reflected sunlight since it is easier to support and move a straight member than a more complex shape.
It is therefore an object of the present invention to provide an efficient solar energy collecting device utilizing a concentrating surface that creates a simply shaped zone of concentration. The zone of concentration follows a predictable path of movement so that movement of the collecting device is significantly simplified.
It is an object of the present invention to provide a concentrator surface that is effective for a long time period each day. The concentration of light is to be received evenly along the length of the absorber so as not to cause hot and cold spots on the absorber.
It is also an object of the present invention to provide a strong and inexpensive roof structure that does not require a special support frame to maintain its shape and desired optical qualities.
Another object of the present invention is to provide a roof structure made up of inexpensive modular panels that feature a non-developable surface which must maintain the complex shape required to optimize the optical qualities of the surface. To successfully function as a roof, the concentrator roof panels must include adequate draining means to permit precipitation to be carried away without interfering with the concentration of sunlight.